A wheeled transport means as specified in the introduction is provided with a suspension system. A suspension system is normally provided for each wheel and usually includes a spring element a damper element, and linkages. These spring, damper and linkage elements define the suspension characteristics and the handling qualities of the transport means, and as such needs to be tailored for the specific transport means.
Additionally the transport means may have a different type of suspension system on each wheel, e.g. on the front compared to the rear wheels.
The main purpose of the suspension system is to ensure continuous grip between the surface upon which the transport means travel and the wheels, and to protect the passengers, cargo and the transport means itself from damage and wear caused by shock and vibration transferred through the wheels and the suspension system to the chassis and caused by unevenness of the surface, which the transport means travel upon.
An important factor is the spring rate, which is defined by the force which the spring exerts divided by the displacement of the spring from equilibrium state. A spring with a small spring rate is “soft” and a spring with a high spring rate is “harder”.
A soft spring needs to be relatively long compared to a hard spring, when carrying the same load in order to avoid that the spring reaches the limit of stroke.
A too soft spring will cause a lag in the control input to the steering of the transport means, which will decrease the handling qualities of the transport means because the steering become sluggish. On the other hand a too hard spring will cause control input with a small lag, which also will decrease the handling qualities of the transport means because the steering become over sensitive.
A too soft spring in a passenger transport means will decrease the comfort of the passengers, because the deflection of the spring is relatively large when subject to an uneven surface, whereas the shock and vibration will remain if the spring is too hard.
Further to the characteristics of the spring; the damper and the physical properties and dimensions of the entire suspension system, is also important, but will not be described further here.
As can be seen from the above the design of a suspension system for a transport means is a trade-off between many design parameters.
From EP 0 246 772 there is known a suspension system with a spring and shock absorber between the vehicle body and each wheel. The shock absorber is electromagnetically switchable between soft and stiff settings. The system employs accelerometers for sensing the vertical acceleration of the vehicle body and control means for switching the setting of the shock absorbers. This suspension system caters for different operating conditions. However, the suspension system is complex and requires the electrical system of the vehicle to perform reliably.
For multipurpose vehicles the design of the suspension system will be a compromise that may not be optimal for the complete operational envelope of the vehicle, unless a very complex suspension system is provided. However, it is desirable to reduce the number of parts in the suspension system especially for multipurpose vehicles to reduce the failure rate.
From U.S. Pat. No. 4,557,500 there is known a suspension system with a reduced parts count consisting of an elastic leaf made of composite material. The elastic leaf may be mounted transversely and supporting two wheels. The elastic leaf element provides the spring of the suspension system. The leaf may consist of fibres extending longitudinally, without being cut, from one end to the other, embedded in an appropriate binder. Although this system provides and improvement with regards to a reduction of parts count the spring characteristics of the suspension does not cater for changing operating conditions.
The problem is not only confined to multi purpose vehicles, but include all transport means, which operate under changing conditions, like trucks and trailers which may operate loaded or empty, a car which need to operate satisfactorily off-road as well as on the road.
US 2006/0032691 A1 disclose a vehicle with a suspension system that is rotatable about a suspension rotation axis between two extreme positions. The wheelbase of the vehicle can hereby be adjusted. When travelling along a horizontal surface the wheelbase is kept at its short setting to maximize the zero turn capability and therefore enhanced manoeuvrability. When the vehicle is travelling along a sloping surface the wheelbase is increased to maintain the overall balance of the vehicle.
U.S. Pat. No. 6,688,586 B1 disclose a leaf spring type suspension system. The leaf spring comprises a plurality of leaf finger that may be pre-tensioned individually by forming the fingers in the free condition. This provides multiple spring rates. The purpose of which is to provide a soft suspension, when the vehicle is lightly loaded and a stiffer suspension, when the vehicle is fully loaded. This system is not suitable for frequent changing of the spring rate, as the leaf fingers are deformed to change the spring rate. Therefore the leaf fingers are prone to fatigue failure.